1. Field of the Invention
This invention relates to a system of detecting optical distortion of a light-transmitting platelike member, in which a predetermined region of the member is divided into a plurality of small parts and a target is sensed in turn through every small part of the region. The invention is particularly, but not exclusively, relates to a system of detecting optical distortion of a front windshield (front window glass) of a vehicle like an automobile.
2. Description of the Prior Art
Recently, the exterior design of an automobile has an inclination to give a complicated three-dimentional curved appearance to the automobile so as to fill aerodynamic requirements, so that the shape of a front windshield (hereinafter described simply as "windshield") has been improved so far. But it is difficult to maintain the surface of the windshield smooth because of a limit of glass forming technique. Accordingly, there are often such cases that an image viewed through the windshield is distorted when a driver sees an outside object. This phenomenon, referred to "optical distortion", is troublesome to the driver if the distortion reaches a certain level, and there is a possibility that the optical distortion obstructs a comfortable drive.
Qualitative evaluation of the optical distortion (which parts have positive lens effect and which parts have negative lens effect) can easily be done if a distribution of light parts and dark parts is examined in a projector photograph. The projector photograph is obtained in such a manner that light of a light source is projected on a screen through the windshield and an image of the windshield produced thereby on the screen is photographed.
When a glass plate is used as the windshield of the automobile, it is, however, necessary to know the optical distortion quantitatively in every part of the glass plate (divided into a plurality of small parts).
In a general way of managing the optical distortion quantitatively, the level of optical distortion is represented by two physical quantities: a lateral distortion angle, a distortion angle measured in a horizontal or lateral direction and a longitudinal distortion angle, a distortion angle measured in a vertical or longitudinal direction with each point of a platelike member such as the windshield. As shown in FIG. 1, the lateral distortion angle is defined by angle .alpha. formed by horizontal line AB with line A'B', which is observed when the line AB is viewed through the platelike member. The longitudinal distortion angle is defined by angle .beta. formed by vertical line AC with line A'C', which is observed when the line AC is viewed through the platelike member. The distortion angles .alpha. and .beta. are substantially in proportion to the amount of the optical distortion.
As another way of quantitatively managing the optical distortion, it is proposed to represent it with two physical quantities of elongation rates: a lateral elongation rate measured in a lateral direction and a longitudinal elongation rate measured in a longitudinal direction at every point of the platelike member, such as the windshield.
In FIG. 1, the lateral elongation rate PL is defined as a ratio (PL=(L.sub.2 -L.sub.1)/L.sub.1) of a difference between length L.sub.1 of horizontal segment AB and length L.sub.2 of segment A'B', to length L.sub.1, and the longitudinal elongation rate PH is defined as a ratio (PH=(H.sub.2 -H.sub.1)/H.sub.1) of a difference between length H.sub.1 of vertical segment AC and length H.sub.2 of segment A'C', to length H.sub.1. The elongation rates PL and PH are substantially in proportion to the amount of the optical distortion.
Shown in FIG. 2 is an illustration in which the distribution of light parts and dark parts represents the lens effect of the windshield. In FIG. 2, the light part corresponds to a region where a positive lens effect (the image of an object is enlarged in lateral, longitudinal or the other directions) was produced and the dark part corresponds to a region where a negative lens effect (the image of the object is contracted in lateral, longitudinal or the other directions) was produced.
According to an illustration shown in FIG. 2, it is, however, impossible to know the amount of the optical distortion quantitatively in every part of the windshield, so that it is difficult to precisely judge whether or not the optical distortion of such windshield raises problems when the windshield is applied to the automobile. Thus, it becomes necessary to examine lateral distortion angle .alpha. and longitudinal distortion angle .beta. in every point of the windshield.
Sometimes, there are such cases as to be impossible to precisely represent the optical distortion shown in FIG. 2 only with the lateral distortion angle .alpha. and longitudinal distortion angle .beta. shown in FIG. 1. The lateral distortion angle .alpha. and longitudinal distortion angle .beta. are produced only in parts of the windshield, in which the lens effect is not constant in the lateral or longitudinal direction. If the lens effect of the part of the windshield is invariably positive in the lateral direction and longitudinal direction, both lateral distortion angle .alpha. and longitudinal distortion angle .beta. are zero as shown in FIGS. 3A and 3B, whereas the lateral elongation is produced as shown in FIG. 3A, and the longitudinal elongation is produced as shown in FIG. 3B. Thus, if only the lateral distortion angle .alpha. and the longitudinal distortion angle .beta. are measured at that time, the measuring result leads to a misjudgment that no optical distortion is produced in the windshield, so that in addition to the measurement of lateral distortion angle .alpha. and longitudinal distortion angle .beta., it is sometimes preferable to measure lateral elongation rate PL and longitudinal elongation rate HL.
Makiguchi et al. research into a determination of "threshold distortion angle" which is an angle that man begins to feel the distortion, by means of combining actually detected distortion angles with sensory evaluation respecting the windshield of the automobile (Makiguchi et al., "Analysis of optical distortion for automobile windshield glass", the fifteen multivariate analysis symposium of Japan Science and Technology Association, November 1991). According to this research, it is proved that the threshold distortion angles differ with each of observation zones G1-G4 (FIG. 4) prescribed as test regions by the Japanese Industrial Standards (JIS) when the windshield 3 is observed from an eye-point prescribed by JIS. Also, it has been made clear that the threshold distortion angles with each of the observation zones G1-G4 can be represented approximately by straight lines (discriminant functions) shown in FIG. 5, if the lateral distortion angle is given on the abscissa, and the vertical distortion angle is given on the ordinate.
In FIG. 5, a driver feels optical distortion if the distortion angles are on the upper sides of the lines that represent the discriminant functions respectively with the zones G1-G4 of the windshield 3, and does not feel any distortion on the lower sides thereof. In zones G1-G3 for example, if the lateral distortion angle is X.degree. and the longitudinal distortion angle is Y.degree. when light passes through each of the small regions of the zones, the driver feels optical distortion, but does not feel in zone G4. Accordingly, by utilizing the discriminant functions, it is possible to evaluate whether man-sensible optical distortion arises or not on the windshield 3, only by detecting the lateral and longitudinal distortion angles when the light passes through each of the many small regions of substantially overall area of the windshield 3.
However, according to Makiguchi et al., the optical distortion of the windshield 3 is detected, as shown in FIG. 6, in a state of mounting the windshield 3 actually on the automobile 9 and fixing a camera at the eye-point (the center of circle 10). The distortion is detected by taking a photograph of a panel 17 surrounding the automobile 9 and having a orthogonal lattice-like pattern, so that the following problems arise.
First, since the distortion angles, which are very little, should be read by man's eyes from the photograph, it takes very long time to be read but accuracy is not so good. Thus, exact detection can not be realized. Furthermore, it takes very long time to do such works as to mount the windshield 3 to the automobile 9, and to take, develop and enlarge photographs, so that it is not suitable for detecting the distortion with the many windshields 3. In addition, a large space is needed, since the panel 17 is large.